As the printing device (ink jetprinter) described above, those of various types of configurations are conventionally known. For example, known is a configuration including a platen for supporting a sheet medium, and a printer head placed above the platen in such a way as to face the platen. A printer head is provided for each color; for example, magenta (M), yellow (Y), cyan (C), and black (K); and then a lot of discharge nozzles are built at a side facing the platen. In the case of a printing device having such a configuration, while making a combination of a motion of a printer head reciprocating from left side to right side and a motion of a medium relatively moving back and forth in a direction perpendicular to the reciprocating direction of the printer head, the printing device takes control of discharging ink (ink drops) from the discharge nozzles toward the medium supported by the platen in order to carry out printing work on a surface of the medium as required.
In recent years, each of the discharge nozzles described above is so shaped as to make its diameter very small in order to enable a delicate color representation by way of discharging fine ink drops. In the meantime, the ink is exposed to air at a tip of the discharge nozzle. If such a condition lasts for a long time, the ink is dried so as to become hardened at the tip of the discharge nozzle, and result in a condition of a clogged nozzle sometimes, in which no ink is discharged from the tip of the discharge nozzle. Especially, in the case of a discharge nozzle shaped so as to have a very small diameter, owing to its structure, the condition of a clogged nozzle is prone to develop easily. If printing operation is carried out under such a condition, a clogged discharge nozzle does not discharge the ink. As a result, there come up missing dots so that it becomes difficult to produce a printed material as required.
In a conventional way of printing work, a certain test pattern is printed on a medium, and then an ink discharge condition of each discharge nozzle (i.e., whether the nozzle is clogged or not) is checked before a start of printing work by means of visually examining the printed test pattern. In ordinary printing work, a white-colored medium (white medium) is often used. Then, if a test pattern is printed on a white medium, as it is, by discharging magenta ink, yellow ink, cyan ink, and black ink, an ink discharge condition can easily be checked since the medium color is different from each ink color.
Recently, developed for more expressive color representation is a printing device provided with a configuration including an additional printer head for discharging white ink (W), besides printer heads for the above 4 colors (magenta, yellow, cyan, and black) (For example, refer to Patent Document 1).